1. Technical Field
The present invention relates to a motor control apparatus for controlling a motor connected to a feed shaft of a machine tool or an industrial machine or an arm of an industrial robot.
2. Description of Related Art
A servo motor is connected to a shaft (mechanical movable part) such as a feed shaft of a machine tool or an industrial machine or an arm of an industrial robot. The rotation of the servo motor is converted to a linear motion of a table, etc., by means of a ball-screw, etc., or the transmission speed of the servo motor is reduced by a speed reducer.
There are cases where a stop position with respect to a certain position of the ball screw or the speed reducer in the positive direction is different from a stop position thereof in the negative direction. In general, this difference is referred to as a backlash which causes a reduction in the positional precision.
FIGS. 8A to 8C are explanatory views of backlash. FIG. 8A illustrates a movable part WA which is moved by a motor (not shown) and a driven part WB which is driven by the movable part WA. The movable part WA is provided, at both ends thereof, with projections A1, and A2, and the driven part WB is provided at its central portion, with a projection B. Therefore, when the movable part WA is moved, for example, in the right direction, the inner end of the projection A1 of the projections A1 and A2 of the movable part WA abuts against one end of the projection B of the driven part WB. As a result, the movable part WA and the driven part WB are moved together in the right direction.
Moreover, when the motor is reversed, the movable part WA is moved in the left direction from the right direction, as shown in FIG. 8B. Consequently, when the inner end of the other projection A2 of the movable part WA abuts against the other end of the projection B of the driven part WB, as shown in FIG. 8C, the movable part WA and the driven part WB are moved together in the left direction.
As may be understood from the foregoing, when the movement is reversed, it is necessary for the movable part WA to move by a predetermined displacement called backlash before the movable part WA abuts against the driven part WB. The backlash C shown in FIGS. 8A and 8C may be a cause of reduction of the positional precision. To prevent this, preparing an amount of compensation corresponding to the backlash C and adding the same to the position command of the motor have been practiced upon reversing the movement.
A semi-closed control device performs a position control of the driven part WB based on the position information of the motor without obtaining the position information of the driven part WB. In such a semi-closed control device, a corrected position command which is obtained by adding a backlash length to the movement command after reversing of the speed command is supplied to the motor, whereby the driven part WB is moved by the displacement corresponding to the movement command.
Moreover, in a device in which both the position information of the motor and the position information of the driven part WB can be obtained, i.e., in a full-closed control device, it is sufficient that a desired value is given as the movement command, because a sensor for the driven part WB is provided. In such a full-closed control device, when the speed command is reversed, the driven part WB begins moving after the motor is moved by a displacement corresponding to the backlash length, and accordingly, a delay in the movement occurs. Therefore, the full-closed control device may have a speed command correcting function to accelerate the motor after the speed command is reversed.
In the two types of control mentioned above, an appropriate compensation is made at an appropriate timing, wherein the amount of compensation and the compensation timing are determined in advance. Therefore, these two types of control both correspond to a feed-forward control.
In contrast thereto, in Japanese Unexamined Patent Publication (Kokai) No. 2014-054001, the position of the motor with backlash is obtained and an amount of compensation of backlash is determined based thereon. In this case, since the amount of compensation of backlash is determined based on the current position of the motor with backlash, the control disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2014-054001 is a feed-back control. In Japanese Unexamined Patent Publication (Kokai) No. 2014-054001, a positional deviation between the movable part and the driven part when the movable part engages with the driven part is referred to as an initial deviation. The amount of compensation is calculated by subtracting the positional deviation between the current positions of the movable part and the driven part from the initial deviation.
In Japanese Unexamined Patent Publication (Kokai) No. 2014-054001, the backlash is interpreted to be merely a gap between tooth surfaces. However, in conventional machines, lost motion occurs due to an elastic deformation caused by a force acting between a movable part and a driven part, in addition to backlash defined merely as a gap. Furthermore, such an elastic deformation could include a linear deformation and a non-linear deformation.
If the lost motion includes an elastic deformation, when the initial deviation is obtained in Japanese Unexamined Patent Publication (Kokai) No. 2014-054001, the magnitude of the force applied to the backlash end and the magnitude of the force required when the motor is reversed differ from one another, and consequently, there is a possibility that the calculated amount of compensation is different from the required amount of compensation.
If the amount of compensation is too small, the workpiece tends to be insufficiently cut. Conversely, if the amount of compensation is excessively large, the workpiece tends to be cut excessively, thus resulting in the formation of over milled. In particular, when the amount of compensation is excessively large, the over milling on the cut surface of the workpiece appears to be a flaw, and consequently, it is highly likely that the workpiece will be judged to have failed to cut, which is often viewed as a problem.
The present invention has been completed in view of the foregoing, and it is an object of the present invention to provide a motor control apparatus which can prevent an excess amount of compensation from being formed, by taking elastic deformation into account.